This invention relates generally to an electromagnetic flowmeter system in which the electromagnet of the flowmeter is excited by a low frequency pulsatory current to produce a signal indicative of flow rate, and more particularly to a converter in a system of this type that automatically damps the output signal in a manner depending on the random noise content of the flow signal.
In a conventional electromagnetic flowmeter, the fluid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the fluid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the fluid and hence to its average volumetric rate, is then amplified and processed to yield an output signal for actuating a recorder or indicator, or for carrying out various process control operations.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is obviated and the A-C flow-induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that are most troublesome are stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and induced loop voltage in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the A-C flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow-induced signal, thereby producing a changing zero shift effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic arrangement adapted to buck out this component, but elimination of its in-phase component has not been successful. Existing A-C operated electromagnetic flowmeters are also known to vary their calibration as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltages both with respect to phase and magnitude. Hence it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered, as previously explained. In order, therefore, to obtain the positive benefits of a steady state field without the drawbacks which accompany continuous d-c operation, the U.S. Pat. No. 3,783,687 to Mannherz et al. discloses an excitation arrangement in which the steady state flux field is periodically reversed or interrupted. The entire disclosure of this patent is incorporated herein by reference.
In the Mannherz et al. patent, in order to avoid the spurious voltages which result from stray couplings without, however, causing polarization of the electrodes, the electromagnet is energized by a low-frequency square wave. This wave is produced by applying the output voltage of an unfiltered full-wave rectifier to the electromagnet and periodically reversing the voltage polarity at a low-frequency rate by means of an electronic switch.
Since the steady state field produced by the square wave is disrupted by switching transients occurring at the points of reversal, the converter to which the signal from the electrodes is applied includes a demodulator which is dated synchronously with the electronic switch to yield an output signal only when the magnetic flux achieves a steady state condition.
While the Mannherz et al. system avoids spurious voltages, it fails to take into account harmonic noise. Because the excitation current for driving the electromagnet has a predetermined frequency, the flow-induced signal yielded by the electrodes contains harmonic noise components which are even and odd harmonics of the drive frequency. These noises result in a less favorable signal-to-noise ratio and impair the reliability and efficiency of the flowmeter.
The Schmoock et al. patent application Ser. No. 967,137, filed Dec. 7, 1978, now U.S. Pat. No. 4,227,408 discloses an electromagnetic flowmeter in which noise components in the signal arising from harmonics of the drive frequency are suppressed to yield an output signal having a favorable signal-to-noise ratio. In this pending application, whose entire disclosure is incorporated herein by reference, the converter includes a pair of synchronous demodulators operating in phase opposition in conjunction with a common integrator to cancel out selected odd and even harmonic noise components.
In a flowmeter of the type disclosed in the Mannherz et al. patent and in the Schmoock et al. patent application, the flow-induced differential signal derived from the flowmeter electrodes is measured during a portion of each "on" (+) and each "off" (-) condition of the magnetic field in the course of an excitation cycle. Successive differences in this signal are taken as a representation of flow rate. Flow signal changes generate output changes, but these cannot exceed the slew time of the instrument. Thus a typical slew time for an instrument of the type disclosed in the above references is 4S (seconds) for a 100% output excursion.
With an instrument of the type disclosed in these references, when operating with a magnet excitation drive frequency of 33/4 Hz, the state of the magnetic field is changed each 133 ms, as a consequence of which the flow signals are updated at 133 ms intervals. A 4S slew time therefore restricts output changes to a maximum of 133/1000.times.100/4 which is equal to 3.33% of full scale for each signal update.
The problem to which the present invention is addressed is random noise, as distinguished from harmonic noise which is the concern of the Schmoock et al. patent application. Random noise which arises from various sources is generally intermittent in nature. A major source of random noise is ionically-charged particles in a slurry or a heavily contaminated fluid being metered. Another source is galvanic noise.
For a flow signal having a significant random noise component, the slew time of the instrument is not sufficient to adequately smooth the output signal, and for this reason it is the usual practice to provide an additional time constant smoothing network. The time constant of this network is a fixed value; and since no two field meter installations are subject to the same conditions of random noise, it is necessary in the field to adjust the time constant to cope with the prevailing noise conditions. In the event these conditions undergo change, one is required to readjust the time constant.
The need to tailor the time constant of a given flowmeter installation to accommodate the system to prevailing noise conditions adds materially to installation and maintenance costs. Moreover, where perceptible changes occur in the random noise component of a flowmeter output signal, and a maintenance man is not available to make the necessary adjustment, the signal having a large random noise content may create problems in industrial process control systems or other apparatus governed by the signal.